For scanning key array in the interface circuit of the conventional dialer, it needs more pins to obtain more keys. Furthermore, it is impossible to have many keys through using different ports between columns or rows.
Please refer to FIG. 1 showing the conventional two-dimensional I/O device. The conventional I/O device includes I/O ports 3 having m columns and n rows, wherein a key 2 can be set between column i (1.ltoreq.i.ltoreq.M) and row j (1.ltoreq.j.ltoreq.n) so that the maximum number of keys 2 is m.times.n. The column group of I/O ports is responsible for outputting the scanning signals and the row group is in charge of detecting keys of the keypad. The external resistors are set between I/O ports and ground wire or in a specific position of I/O ports so that the number of the external resistor is equal to that of I/O ports. However, it will waste a lot of space over column i and row j because a key can not be set between two columns or two rows. Therefore, the maximum number of external resistors 1 to be detected is m+n in the conventional two-dimensional I/O device. In general, a telephone has more than five or six kinds of communication specifications and also includes some specific functions supplied by manufacturers so that the external resistors 1 are not sufficient for applications. The designs for the conventional two-dimensional I/O device mostly use an oscillator of 3.58 MHz as a source of system frequency. However, the oscillator of 3.58 MHz has a substantial power comsumption.